Joint replacement implants have been developed for the major skeletal joints of the human body. The implants are used to replace diseased or traumatized joint surfaces to reduce pain and restore function. Such implants typically include opposing joint components wherein each component defines an articulating bearing surface. For example, an orthopedic knee implant may include a femoral component for replacing the femoral articulating condyles and a tibial component for replacing the tibial articulating surface. The femoral component typically includes a metallic articulating bearing surface which glides and pivots on a non-metallic articulating bearing surface of the tibial component. For example, many knee implants comprise a femoral component made of a metal such as Ti-6Al-4V alloy or Cobalt-Chromium-Molybdenum alloy and a tibial bearing surface made of ultra high molecular weight polyethylene (UHMWPE). Whatever type of implant or materials are involved, it is generally desirable for orthopedic bearing surfaces to exhibit low wear. Furthermore, the bearing surfaces have minimum strength and toughness requirements. These bearing surfaces primarily experience compressive loading and sliding wear.
In addition to providing for articulation, the implant components may include constraint mechanisms to provide some degree of constraint to the motion permitted between the components. These constraint mechanisms may replace or augment natural constraint in the joint such as that provided by tendons and ligaments crossing the joint. These constraint mechanisms may provide the function of missing or damaged natural constraints. For example, in a posterior stabilized knee prosthesis, the tibial articular surface may include a post that projects upwardly to interact with a cam and box formed on the femoral component to limit rotation, varus/valgus tipping, anterior/posterior translation, posterior rollback, and/or other parameters of knee motion. Such constraint mechanisms may see significant bending, shear, and even impact loads within the limits of normal operation. Therefore, it is generally desirable for the constraint mechanism components to have high strength and toughness. Furthermore, the constraint mechanisms have limits to the permissible wear that can occur between contacting surfaces.
It may be the case that a particular type of implant has an articular bearing surface for which low wear is of prime importance and which further includes a constraint mechanism portion for which strength and toughness are of prime importance. For example, in a posterior stabilized knee, it is desirable for the articular condyles to exhibit low wear and the tibial post to exhibit high strength and toughness.